This invention relates to methods of joining superconducting tapes, and more specifically, to methods of forming a superconducting joint between superconducting tapes. As used herein, the term "tape" means an elongate body having major surfaces in the width and length dimensions, and a small dimension, i.e., the thickness.
Superconductivity is that characteristic of certain materials which permits them to conduct electric currents without resistance. A superconducting material exhibits this characteristic only when its temperature is below the superconducting critical temperature of the material and then only if it is not subject either to a magnetic field greater than the superconducting critical magnetic field of the material or to an electric current greater than the superconducting critical current of the material. Accordingly, superconductivity can be quenched, i.e., returned to a resistive state, by increasing the temperature, magnetic field, or current to which the superconducting element is subjected above the critical temperature, magnetic field, or current. Quenching of the superconductivity may occur abruptly or more gradually depending upon the particular material, i.e., the relative breadth of its superconducting transition state in terms of temperature, magnetic field, or current.
Superconductive bodies of laminated construction having an elongated tape or strip configuration and the methods of producing such superconductive tapes are well known. For example, British patent 1,254,542 incorporated by reference herein, discloses improved superconducting tapes, and methods of forming the improved tapes. U.S. Pat. No. 3,537,827, incorporated by reference herein, discloses improvements in laminating superconductive tapes and methods for producing the laminated tapes.
Briefly stated, it is known that selected parent-metals, either pure or preferably containing minor alloying additions, are capable of being reacted with other metals and forming superconducting compounds or alloys that have a high current carrying capacity. Parent-metals niobium, tantalum, technetium, and vanadium can be reacted or alloyed with reactive-metals tin, aluminum, silicon, and gallium to form superconducting alloys, such as triniobium tin. As used herein, the term "triniobium tin" is a superconducting alloy in the form of an intermetallic compound comprised of three niobium atoms per tin atom.
Additionally, it is understood that the superconductive alloys or compounds can be improved by first alloying the parent-metal, i.e , niobium, tantalum, technetium, and vanadium with a minor amount of a solute metal having an atom diameter of at least 0.29 angstrom larger than the diameter of the parent-metal atom. A broad disclosure of various parent-metals, solute metals, and reactive-metals can be found in U.S Pat. No. 3,416,917. U.S. Pat. No. 3,429,032 discloses improved critical currents in triniobium tin superconducting alloy formed when niobium containing zirconium up to about 25 percent is heated in the presence of excess tin, and a non-metal selected from the group consisting of oxygen, nitrogen, and carbon.
It is also known that the reactive-metals can be alloyed to improve the superconductive tape. For example, the critical current density of triniobium tin has been improved by making copper additions in the reactive-metal tin for coating on niobium tape as disclosed in, "Enhancement of the Critical Current Density in Niobium-Tin" J. S Caslaw, Cryogenics, February 1971, pp. 57-59 As used herein, the term "reactive-metals" includes the alloys of the metals tin, aluminum, silicon, and gallium that react with parent-metals to provide superconductive alloys, for example, a tin alloy comprised of up to 45 weight percent copper.
It has been found that niobium is an important parent-metal due to the superior superconducting alloys which it will form. For example, small percentages generally greater than one-tenth weight percent of a solute metal can be added to the niobium parent-metal to effectively increase its current carrying capacity. Zirconium additions are felt to be those most advantageous. The solute materials, for example, zirconium, are added in amounts up to about 33 atomic percent. Other solute additives are used in similar amounts.
The solute-bearing niobium is reacted with either tin, aluminum, or alloys thereof by contacting the niobium with either of these metals or alloys, and then heating them to an elevated temperature for a time sufficient to cause suitable reaction to occur. Especially advantageous materials are those of the niobium-tin compositions in which the ratio of niobium to tin approximates three to one, i.e., triniobium tin, since these materials have superior superconducting properties.
The triniobium tin alloy has been fabricated in various forms, particularly wires and tapes, in efforts to produce devices such as high field superconducting electromagnets. One method for obtaining superconducting tape in a continuous fashion is that wherein a tape of a preselected parent-metal, such as niobium or niobium alloy, is continuously led through a bath of molten reactive-metal such as tin or tin alloy. The tape picks up a thin coating of the reactive-metal from the molten bath and the tape is subsequently heated in a reaction furnace to cause formation of a superconductive alloy on the surface of the parent-metal tape.
The superconducting alloy formed on the tape is fragile, and outer laminae of non-superconductive metal are applied to the tape to make a laminated superconductor that is strong and capable of being wound onto coils without damage to the superconductive material. For example, a relatively thin tape of niobium foil is treated with tin to form an adherent layer of triniobium tin on the surfaces of the tape, and copper tapes of substantially the same width are soft soldered to each of the major surfaces of the superconductive tape to form a symmetrically laminated structure. Because of the difference in the coefficient of thermal expansion of copper and the niobium-niobium tin material, the brittle intermetallic compound is placed in compression even at room temperature, minimizing the danger of mechanical fracture when coiling.
One use for such superconductive tape is for the windings in superconducting magnets. For example, a magnetic resonance imaging device can use 6 superconducting magnets, with the windings in each magnet requiring a continuous length of superconducting tape of over a kilometer. Individual magnets in the device are connected together to provide a continuous superconducting path through all six magnets. As a result, a continuous length of superconducting tape of many kilometers would be required for the device. Continuous lengths of many kilometers of superconductive tape are not currently available, and many shorter lengths would have to be joined. In addition, it can be expected that some breakage and damage of the tapes will occur during tape winding operations, necessitating joints to repair such breakage or damage.
Superconducting magnets are often used in apparatus requiring a constant magnetic field from the magnet. To maintain the constant magnetic field the magnet must operate in the superconducting, or persistent mode. Current loss in the magnet from internal resistance causes drift or reduction of the magnetic field. As a result, a superconducting joint is desirable for making the necessary connections between superconducting tapes to prevent drift of the magnetic field. The current carrying capacity and magnetic field behavior of the joints should at least approach the current carrying capacity and magnetic field behavior of the superconducting tape, or the joints will become the limiting factor in the current carrying capacity of the device.
An object of this invention is a method for forming superconducting joints between superconducting tapes where the joints have a high current carrying capacity, approaching the current carrying capacity of the superconducting tape.
Another object of the invention is a method for forming superconducting joints between superconducting tapes where the joints sustain the superconductive properties in high magnetic fields approaching the high field behavior of the superconductive tape.